Method of manufacturing a projection objective and projection objective

ABSTRACT

The disclosure relates to a method of manufacturing a projection objective, and a projection objective, such as a projection objective configured to be used in a microlithographic process. The method can include defining an initial design for the projection objective and optimizing the design using a merit function. The method can be used in the manufacturing of projection objectives which may be used in a microlithographic process of manufacturing miniaturized devices.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims benefit under 35 USC120 to, International Application No. PCT/EP2007/010244, filed Nov. 26,2007, which claims benefit of U.S. Ser. No. 60/861,713, filed Nov. 30,2006 and European Application No. 06024789.7, filed Nov. 30, 2006.International Application No. PCT/EP2007/010244 is hereby incorporatedby reference.

FIELD

The disclosure relates to a method of manufacturing a projectionobjective, and a projection objective, such as a projection objectiveconfigured to be used in a microlithographic process. The method caninclude defining an initial design for the projection objective andoptimizing the design using a merit function. The method can be used inthe manufacturing of projection objectives which may be used in amicrolithographic process of manufacturing miniaturized devices.

BACKGROUND

Microlithographic processes are commonly used in the manufacture ofminiaturized devices, such as integrated circuits, liquid crystalelements, micro-patterned structures and micro-mechanical components. Insuch processes, a projection objective projects patterns of a patterningstructure (usually a photo mask (mask, reticle)) onto a substrate(usually a semiconductor wafer). The substrate is coated with aphotosensitive layer (resist) which is exposed with an image of thepatterning structure using projection radiation.

Generating a new design of a projection objective can be a complicatedtask involving an optimization of structural parameters and qualityparameters of the projection objective. The structural parametersinclude refractive indices of materials of which the lenses are formed,surface shape parameters of lenses and mirrors (if applicable),distances between first and second surfaces of each lens, distancesbetween surfaces of different optical elements, a distance between theobject plane of the projection objective and an entry surface of theobject-side front element of the projection objective, a distancebetween an exit surface of an image-side front element of the projectionobjective and the image plane, refractive indices of media disposedbetween adjacent optical elements, between the object plane and theobject-side front element and between the image plane and the image-sidefront element. Quality parameters include parameters describing theoptical performance of the projection objective e.g. in terms ofselected aberrations, image-side numerical aperture, magnification ofthe projection objective and the like.

The optimization of a design to conform to a desired specification ofthe optical performance and other quality features of the projectionobjective typically involves computational methods such as ray tracingto optimize the parameters of the projection objective while observingcertain boundary conditions. CODE V, a lens analysis and design programsold by Optical Research Associates, Inc., is a commonly used softwaretool employed for that purpose. The optimization includes minimizing ormaximizing a suitably chosen merit function depending on the parametersof the design. Typically, the merit function construction is done byutilizing several merit function components, which may represent opticalaspects, manufacturability aspects and other aspects describing theoptimization goal of the specific design.

SUMMARY

In some embodiments, the disclosure provides a method of manufacturingprojection objectives that allows to manufacture complex projectionobjectives for microlithography in a cost effective way whilemaintaining high standards with respect to optical performance.

In certain embodiments, the disclosure provides a method ofmanufacturing projection objectives that allows to manufactureprojection objectives for microlithography having an optical performancewhich is relatively insensitive with respect to surface imperfectionspresent on optical surfaces within the projection objective.

In some embodiments, the disclosure provides a method of manufacturingprojection objectives having a low level of field variation ofintensity.

In certain embodiments, the disclosure provides a projection objectivewhich is relatively insensitive towards surface imperfections caused bycontamination and other effects on optical surfaces within theprojection objective.

In some embodiments, the disclosure provides a method of manufacturing aprojection objective. The method can include defining an initial designfor a projection objective and optimizing the design using a meritfunction. The method can further include defining a plurality of meritfunction components, each of which can reflect a particular qualityparameter. One of the merit function components can define a maximumdesired irradiance involving a normalized effective irradiance valuerepresenting an effective irradiance normalized to an effectiveirradiance in an image surface of the projection objective does notexceed a predefined irradiance threshold value on each optical surfaceof the projection objective except for a last optical surface directlyadjacent to an image surface of the projective objective. The method canalso include computing a numerical value for each of the merit functioncomponents based on a corresponding feature of a preliminary design ofthe projection objective, and computing from the merit functioncomponents an overall merit function expressible in numerical terms thatreflect quality parameters. In addition, the method can includesuccessively varying at least one structural parameter of the projectionobjective and recomputing a resulting overall merit function value witheach successive variation until the resulting overall merit functionreaches a predetermined acceptable value. Further, the method caninclude obtaining the structural parameters of the optimized projectionobjective having the predetermined acceptable value for the resultingoverall merit function. The method can also include implementing theparameters to make the projection objective.

The term “irradiance” describes the power of electromagnetic radiationincident on the surface. The term “effective irradiance” as used herindescribes the contribution to an overall irradiance incident on anoptical surface, which contribution originates from radiation emergingfrom one single object field point. The effective irradiance isalternatively denoted as “pin-hole irradiance” in this application.

Where large values of effective irradiance are avoided on opticalsurfaces, the optical performance of a projection objective may be maderelatively insensitive with respect to surface imperfections present onoptical surfaces within the projection objective.

In many cases, the “last optical surface”, i.e. the optical surface ofthe projection objective closest to the image surface, is subject toparticular conditions involving the last optical surface to be excludedfrom the optimization process. The spatial distribution of points ofincidence of radiation rays across the last optical surface is largelyinfluenced by the concept of the projection process involved (dryprojection or immersion projection) and on the geometrical conditions inthe image side working space (space between the last optical surface andthe image surface, where the substrate surface is to be placed.). Forexample, it may be useful to provide a narrow gap (typically one or moremillimeters wide) between last optical surface and image surface tointroduce an immersion liquid for immersion exposure. In view of theinflow and outflow of immersion medium in the image space it is oftendesired to have an essentially planar last optical surface. Under theseconditions, the intensity load of the last optical surface (representede.g. by the effective irradiance) is basically determined by theimage-side working distance, the size of the effective image field, therefractive index of the medium after the last optical element, and theimage side numerical aperture NA. Therefore, the structural parametersdescribing position and surface shape of the last optical surface are nofree parameters and should be excluded from the optimization procedure.

Various conditions may cause or contribute to local maxima of effectiveirradiance on optical surfaces. For example, large effective irradiancevalues may occur where an optical surface lies within a caustic region.In some embodiments, those conditions are systematically avoided bycalculating a position and an extent of potential caustic regions withinthe projection objective; and by optimizing the structural parameters ofthe projection objective such that no optical surface is positionedinside a caustic region.

Alternatively, or in addition, optical surfaces may be relativelyimportant where the size of real sub-apertures of ray bundles becomesrelatively importantly small. Therefore, in certain embodiments, themethod includes: defining a number of representative field points;calculating ray bundles originating from the field points andintersection zones of the ray bundles with optical surfaces, where anintersection zone of a ray bundle with an optical surface defines a realsub-aperture having a sub-aperture size defined by the area of theintersection zone; defining a sub-aperture size threshold value; andoptimizing the structural parameters of the projection objective suchthat the real sub-aperture size for selected field points does not fallbelow the sub-aperture size threshold value for all optical surfaces ofthe projection objective except for a last optical surface directlyadjacent to an image surface of the projection objective.

Embodiments may include routines considering more than one cause ofrelatively importantly high effective irradiance concentrations. Forexample, one merit function component may define a desired maximumirradiance as described above, and another merit function component maydefine a minimum real sub-aperture property such that a resultingoptical design will automatically have only optical surfaces where thesize of real sub-apertures on all optical surfaces (except for lastoptical surface) lies above a predefined real sub-aperture sizethreshold value.

The method may be applied in the design of various types of opticalsystems, such as projection objectives used for microlithography havinga relatively high image-side numerical aperture, as frequently found inprojection objectives for immersion lithography, where NA>1 may beobtained. A detailed analysis of a large number of prior art projectionobjectives has shown that small effective and/or real sub-aperturesand/or caustic conditions may occur particularly in catadioptricprojection objective having at least one concave mirror and at least oneintermediate image as well as at least one planar deflecting mirror toseparate the beam bundle running towards the concave mirror from a beambundle reflected from the concave mirror. Projection objectives of thistype may be designed such that they have no optical surface (except forthe last optical surface adjacent to the image surface) where causticconditions occur and/or where relatively importantly small effectiveand/or real sub-apertures occur.

In some embodiments, a projection objective includes a plurality ofoptical elements arranged to image an off-axis object field arranged inan object surface onto an off-axis image field arranged in an imagesurface of the projection objective. The optical elements form: a firstrefractive objective part generating a first intermediate image fromradiation coming from the object surface and including a first pupilsurface; a second objective part including at least one concave mirrorimaging the first intermediate image into a second intermediate imageand including a second pupil surface optically conjugated to the firstpupil surface; and a third refractive objective part imaging the secondintermediate image onto the image surface and including a third pupilsurface optically conjugated to the first and second pupil surface.

Embodiments may have exactly two intermediate images.

The second objective part may have exactly one concave mirror and theprojection objective may have a first folding mirror for deflectingradiation coming from the object surface in the direction of the concavemirror, and a second folding mirror for deflecting radiation coming fromthe concave mirror in the direction of the image surface. The deflectingmirrors may both be planar. The projection objective may be designed forimmersion lithography at NA>1.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show prior art catadioptric projection objectives havingexactly two intermediate images and a single concave mirror;

FIG. 2 shows a schematic drawing of an axial view of a lens surfacepositioned at a pupil surface and illuminated with dipole illumination;

FIG. 3 shows a schematic flow chart illustrating a manufacturing methodto avoid optical surfaces having relatively importantly largeconcentrations of irradiance;

FIG. 4 shows a pupil raster of pupil points to divide a pupil surfaceinto raster fields having substantially the same raster field area;

FIG. 5 shows intersection points of rays of the pupil raster shown inFIG. 4 on a selected optical surface distant from the pupil surface,where a region of relatively high effective irradiance (or pin-holeirradiance) is illustrated;

FIG. 6 shows intersection points of rays of the pupil raster shown inFIG. 4 on a selected optical surface distant from the pupil surface,where a region with caustic condition is illustrated;

FIG. 7 shows a catadioptric projection objective having two intermediateimages, one concave mirror and two planar deflecting mirrors, wherecaustic conditions are systematically avoided on optical surfaces closeto the intermediate images; and

FIG. 8 shows footprints of selected field points around the edge of arectangular field on the first and second folding mirrors of FIG. 7.

DETAILED DESCRIPTION

As an introduction to some of the embodiments of the disclosure considera high aperture catadioptric projection objective adapted for immersionlithography NA>1. In many cases such designs exhibit rather small realsub-apertures on optical surfaces. As used herein, the term “realsub-aperture” refers to a “footprint” (intersection zone) of a raybundle originating from a specific object point (i.e. field point in theobject surface) on an optical surface. Small real sub-apertures mayoccur particularly in catadioptric projection objectives having at leastone concave mirror and at least one intermediate image as well as atleast one planar deflecting mirror to separate a beam bundle runningtowards the concave mirror from a beam bundle reflected from the concavemirror.

For illustration purposes, FIG. 1A shows a detail of a prior artcatadioptric projection objective taken from FIG. 2 of WO 2004/019128A2. The projection objective includes the first refractive objectivepart OP1 to image an object field from the planar object surface OS intoa first intermediate image IMI1, a second, catadioptric objective partOP2 including a concave mirror CM for imaging the first intermediateimage IMI1 into a second intermediate image IMI2, and a third,refractive objective part OP3 (only partially shown) to image the secondintermediate image IMI2 into the planar image surface, which is parallelto the object surface OS. A first planar folding mirror FM1 deflectsradiation coming from the object surface towards the concave mirror CM.A second planar folding mirror FM2 at right angles to first foldingmirror deflects radiation coming from the concave mirror towards theimage surface.

A ray bundle RB originating from an off-axis field point FP1 of aneffective object field positioned entirely outside the optical axis AXintersects the optical surfaces in intersection zones of varying sizedepending on the position of the optical surface within the opticalsystem. Those intersection zones are denoted “real sub-apertures” inthis application. Some representative intersection zones are emphasizedin bold lines in FIG. 1. The first real sub-aperture SA1 on the entryside of a plane-parallel plate directly following the object surface israther close to a field surface (object surface) and relatively small.The size of a second real sub-aperture SA2 close to a first pupilsurface P1 on a concave entry side of a meniscus lens essentiallycorresponds to the size of the pupil at that position and is relativelylarge. All real sub-apertures substantially overlap at the pupilsurface. Real sub-apertures become increasingly smaller as the opticalsurfaces are positioned closer to the first intermediate image IMI1, ascan be seen for real sub-aperture SA3 on the concave exit side ofpositive meniscus lens immediately upstream of the first folding mirrorFM1, and by real sub-aperture SA4 formed on the first folding mirror FM1immediately upstream of the first intermediate image. In contrast, afifth real sub-aperture SA5 on a convex surface of a negative meniscuslens immediately in front of the concave mirror CM has a large sizeessentially corresponding to the size of the second pupil P2 where theconcave mirror CM is positioned.

An off-axis object field and image field is used in those designs toobtain an image free of vignetting and pupil obscuration. When anoff-axis field is used, the efforts involved for correcting imagingaberrations increase with increasing distance between the off-axisobject field and the optical axis such that the size of the “designobject field” increases. The “design object field” includes all fieldpoints of the object surface which can be projected by the projectionobjective with an imaging fidelity sufficient for the intendedlithographic process. All imaging aberrations are corrected sufficientlyfor the intended projection purpose inside the design object field,whereas at least one of the aberrations is higher than a desiredthreshold value for field points outside of the design object field. Inorder to facilitate correction, it may therefore be desirable to keepthe size of the design object field small, which, in turn, involves tominimize the offset between the optical axis and the off-axis objectfield. Efforts to minimize this offset often lead to designs havingmirror surfaces and/or lens surfaces relatively close to a fieldsurface, whereby the corresponding real sub-apertures present on thoseoptical surfaces close to field surfaces become small. For example, thereal sub-apertures SA3 and SA4 on lens and mirror surfaces immediatelyupstream of first intermediate image IMI1 are relatively small.

Small real sub-apertures on optical surfaces generally involve severedesired properties with respect to cleanliness of the manufacturingprocess and the resulting optical surfaces, which should be essentiallyfree of significant surface imperfections. As the energy emitted from aselected field point is concentrated on a relatively small area in aregion of a small real sub-aperture, an imperfection present in thatregion of small real sub-aperture would block some of the lightintensity present in the ray bundle, thereby creating light loss. Wherethe corresponding real sub-aperture is small, a surface imperfection ofa given size causes a larger light loss effect than in an area with alarger real sub-aperture simply because the ratio between the area ofthe imperfection causing the perturbation and the area of the realsub-aperture (footprint area) becomes more unfavourable. Localized lightloss caused by imperfections may cause uniformity errors with directproblems regarding the critical dimensions of structures manufacturedwith a projection objective. Specifically, undesirable variations of thecritical dimension (CD variation) may be caused in connection with thesensitivity level of the light-sensitive coating (photoresist) on asubstrate.

Further, localized light loss may also cause telecentricity errorsresulting from a shift of an energetic centroid of the pupil generallyin direction of the undisturbed areas. As an illustration, FIG. 2 showsschematically an axial view of lens surface LS positioned at a circularpupil surface in a projection objective. The projection exposure machineis operated with dipole illumination to improve, for example, depth offocus and/or contrast when printing densely packed unidirectional lines.The corresponding intensity distribution of projection radiation ischaracterized by two “poles”, a first pole PO1 and a second pole PO2, onopposite sides of the optical axis AX, where light energy isconcentrated. No light intensity is on the optical axis. A surfacecontamination CON is present on the lens surface LS in the area of firstpole PO1. As the contamination CON blocks a considerable fraction oflight energy present in the first pole PO1, the energetic centroid ofthe pupil will be shifted towards the undisturbed second pole PO2. Thisdecentred energetic centroid in the pupil corresponds to a propagationdirection in which the intensity centroid of the image propagates in theimage space where the wafer is positioned. Due to the contamination CON,the propagation direction deviates from the optical axis and is directedat a finite angle thereto. An oblique orientation of the aerial imageresults, which may lead to telecentricity-induced distortion for exampleif the position of the substrate surface varies locally due to an unevensubstrate surface and/or due to positioning variations caused by thewafer stage.

The size of the real sub-apertures as mentioned above is considered as agood indicator showing which optical surfaces might be relativelyimportant with respect to surface imperfections if it is assumed thatthe real sub-apertures are illuminated substantially homogeneously suchthat there are no or very little differences in local radiation energyinflow between different locations within a real sub-aperture.

A substantially homogeneous distribution of irradiance within realsub-apertures may be a good approximation in many purely refractiveoptical systems. However, there may be significant variations in localirradiance within a real sub-aperture such that significant differencesin local irradiance may occur within the real sub-aperture. In order toaccount for possible inhomogeneities regarding local irradiance withinreal sub-apertures it has been found useful to define an “effectivesub-aperture”, which allows to account for local variations ofirradiance within real sub-apertures. As noted above, imperfections onoptical surfaces may be relatively important where relatively largeradiation energy is concentrated in the area of an imperfection.Therefore, within a real sub-aperture those regions may be particularlyimportant where the local irradiance has a relatively large value whencompared to other parts of the real sub-aperture. In order to largelyavoid problems associated with local concentrations of radiation energy,it is considered useful to analyse the area of a real sub-aperture inorder to identify the region (or regions) where a maximum value for theeffective irradiance occurs. In a geometrical picture, the localirradiadiance may also be characterized by a specified value for a“geometrical ray density” of rays originating from a single field pointin the object plane, where a maximum value for the effective irradianceoccurs where a maximum value for the geometrical ray density occurs.

Now consider a system optimization which accounts for those local maximaof geometrical ray density (or effective irradiance). The tolerances forthe system layout may have a certain value assuming that each realsub-aperture is homogeneously illuminated. However, the tolerances haveto be tighter where local concentrations of radiation energy might occurwithin a real sub-aperture. This effect may be accounted for by definingan “effective sub-aperture”, which corresponds to the sub-aperture whichrepresents the location with maximum geometrical ray density (or maximumeffective irradiance). Within this concept, the size of the “effectivesub-aperture” equals the size of the “real sub-aperture” in systemswhere the real sub-aperture is homogeneously illuminated. However, iflocal concentrations of radiation energy occur within the realsub-aperture, this is expressed by the size of the effectivesub-aperture, which then becomes smaller than the size of the realsub-aperture.

By way of illustration, FIG. 1B shows a detail of another prior artcatadioptric projection objective taken from embodiment 5 of WO2004/019128 A2. Identical or similar features are designated withidentical reference numerals as in FIG. 1A. One ray bundle RB emergesfrom a selected field point FP1 in the object surface OS. The openingangle of the ray bundle at the object surface is determined by theobject-side numerical aperture NA_(OBJ). The trajectories of variousselected rays including rays R1 and R2 are shown. As evident from thedrawing, the geometrical ray density defined by the rays of a ray bundleRB is essentially homogeneous across the optical surfaces up to theregion of the large positive meniscus lens ML upstream of first foldingmirror FM1. However, as the rays approach the first folding mirror FM1,the local density of rays (i.e. the geometrical ray density) increasesin the outer part of the ray bundle (farthest away from the optical axisAX) when compared to the region closer to the optical axis. With otherwords, the geometrical ray density becomes inhomogeneous in the regionof the first folding mirror a small distance upstream of the firstintermediate image IMI1. Specifically, rays R1 and R2 originating fromfield point FP1 at different aperture values intersect in the regionclose to or at the first folding mirror FM1. (Note that thoseintersections of single rays occur particularly in the regions of firstfolding mirror FM1 and second folding mirror FM2 optically close to theintermediate images IMI1, IMI2, respectively.) The intersection of raysoriginating from a common field point at different apertures indicatesthe existence of a “caustic” condition. It is apparent from FIG. 1B thatboth the first folding mirror and the second folding mirror FM1, FM2 arepositioned in regions where rays of the ray bundle RB intersect, i.e.are positioned in “caustic regions”.

The geometrical ray density corresponding to a particular field point,as described above, may be considered as an expression for thecontribution of a particular field point to the total irradiance on anoptical surface. This contribution is denoted “effective irradiance” inthe present application. In a mental experiment, consider an illuminatedpinhole (representing one single field point) in the object plane of theoptical system. A ray bundle originates from that field point. The“effective irradiance” corresponding to that field point is theirradiance contribution of the ray bundle originating from that fieldpoint on a selected optical surface. This contribution to the overallirradiance incident on the optical surface is a function of the position(or location) on the optical surface as well as a function of theposition of the pinhole in the object surface. It is contemplated thatthe maximum value of all values of effective irradiance (irradiancecorresponding to a single object field point) should not exceed apredetermined “irradiance threshold value”.

In a caustic region, i.e. in a region where caustic conditions exist,the effective irradiance (pin-hole irradiance) may become divergentpractically leading to a severe concentration of light energy on arelatively small surface area. In terms of ray propagation, a causticcondition is given on an optical surface if different rays emitted froman object point at different numerical aperture intersect on the opticalsurface or in the vicinity thereof. A surface imperfection on an opticalsurface positioned in a caustic region has an influence on rays emittedfrom an object point at different aperture angles, thereby potentiallydeteriorating imaging quality substantially more than an imperfectionplaced in a region outside a caustic region.

In some embodiments of a method of manufacturing a projection objective,the occurrence of caustic conditions on optical surfaces and/or ofrelatively importantly small effective sub-apertures on optical surfacesis systematically avoided due to a corresponding sub-routine in thesoftware used for the computer-based optical design leading to thegeneral layout of the optical elements of the projection objective.Where those relatively important conditions are systematically avoided,the desired properties with regard to cleanliness of the process can berelaxed without substantially jeopardizing the optical performance ofthe resulting projection objective.

In the embodiment, one of the merit function components used in thecomputational process defines a desired maximum irradiance propertywhich involves that the maximum value for the effective irradianceIRRDD_(EFF) occurring on each optical surface (except for the lastoptical surface closest to the image surface) does not exceed apredetermined irradiance threshold value IRRTV.

As used here, the term “irradiance” describes the power ofelectromagnetic radiation at a surface, per unit area. Specifically, theterm “irradiance” describes the power of electromagnetic radiationincident on the surface. The SI units for irradiance is Watts per squaremeter (W/m²). The irradiance is sometimes also called intensity, but maynot be confused with a radiant intensity, which has different units. Theterm “effective irradiance” (also denoted as “pin-hole irradiance”)describes the contribution to an overall irradiance incident on anoptical surface, which originates from radiation emerging from onesingle object field point. In terms of ray propagation, the “effectiveirradiance” corresponds to a “geometrical ray density” of different raysoriginating from a common field point at equidistant aperture steps. Theeffective irradiance may be homogeneous on a surface. In general, theeffective irradiance may vary across a surface to define a region ofmaximum effective irradiation (corresponding to large local geometricalray density) and regions of smaller values for effective irradiation(corresponding to smaller geometrical ray densities).

Where relatively large values of effective irradiance occur on anoptical surface, those optical surfaces may be important with respect tosurface imperfections, such as scratches and contamination. Relativelylarge values of effective irradiance may be caused, for example, bycaustic conditions on optical surfaces and/or by relatively importantlysmall effective sub-apertures on optical surfaces

Embodiments of an optimization routine implemented into a softwareprogram used to calculate an optical design (layout) of projectionobjective is now explained in context with the schematic flow chartshown in FIG. 3. In a first step S1 (Optimization with regard toaberrations, OPT AB) a basic layout of an optical design is optimizedwith respect to aberrations to obtain an optimized design D1. To thisend, conventional subroutines of a suitable optical design program maybe used to vary one or more structural parameters of the projectionobjective and to calculate the resulting overall aberrations of thedesigns. The resulting optimized design D1 may or may not have opticalsurfaces where large concentrations of effective irradiance occurlocally on one or more optical surfaces.

In a second step S2 the optimized design is analysed to determinewhether or not a normalized effective irradiance value IRRAD_(EFF)representing an effective irradiance normalized to an effectiveirradiance in an image surface of the projection objective exceeds apredefined irradiance threshold value IRRTV on each optical surface ofthe projection objective (except for the last optical surface directlyadjacent to the image surface of the projection objective).

If the normalized effective irradiance value IRRAD_(EFF) does not exceedthe irradiance threshold value IRRTV on any optical surface (with apossible exception for the last optical surface), then the parametersdescribing the optimized design D1 are input into an aberrationdetermination step S3 to determine whether or not the aberration levelAB lies above or below a predefined aberration threshold value ATV. Ifthe aberration level is below the aberration threshold value, then anoptimized design D2 is output as a result of the optimization procedure.Final design D2 may be identical to optimized design D1 which served asan input for the irradiance determination in step S2.

If the irradiance determination in step S2 determines that thenormalized effective irradiance value IRRAD of optimized design D1exceeds the irradiation threshold value IRRTV for at least one opticalsurface (except for the last optical surface), then the calculationroutine proceeds to irradiance optimization step S4 (OPT IRRAD), wherethe optimized design D1 is reoptimized with regard to effectiveirradiance to reduce local concentrations of irradiance on relativelyimportant optical surfaces. To this end, at least one structuralparameter of the projection objective is varied and an overall meritfunction including the merit function component defining the desiredmaximum irradiance property are employed. The resulting reoptimizeddesign D1′ is then input via step S1 into the irradiance determinationstep S2 to determine whether or not the normalized irradiance value isnow below the irradiance threshold value IRRTV for each of theconsidered optical surfaces. An iteration including two or morereoptimization steps may be used for that purpose.

If the irradiance determining step S2 determines that the resultingreoptimized design D1′ is optimized with regard to the occurrence oflocal maxima of irradiance, then the optimized structure is input intothe aberration determination step S3 to determine whether thereoptimization with regard to irradiance has caused one or morerelatively important aberrations to lay above a respective aberrationthreshold value. If the reoptimized design is still acceptable withregard to aberrations, a final design D2′ is obtained and output in stepS5.

If the design needs to be optimized with regard to aberrations, thestructural parameters received from step S2 are input into step S1 tomodify the optical design (which has already been optimized with regardto irradiance) also with regard to aberrations. One or more iterationsmay be involved to reoptimize the design with regard to aberration andstill obtain normalized irradiance value below a particular irradiancethreshold value IRRTV for all relatively important optical surfaces. Afinal design D2′ is the result of the reoptimization procedure.

In some embodiments the subroutine for the irradiance determiningdetermination step S2 allows to avoid the occurrence of causticconditions on all optical surfaces and/or to avoid relativelyimportantly small sub-apertures on all optical surfaces except for thelast optical surface closest to the image surface.

In certain embodiments of a computational routine designed tosystematically avoid the occurrence of caustic condition on opticalsurfaces is now explained in connection with FIG. 4 to 6.

In a first step of the computational part of the manufacturing method anumber of representative field points is defined. Ray tracing isperformed for rays of ray bundles originating from those representativefield points.

In a second step a pupil raster is defined, where the pupil rasterrepresents an array of raster points in a pupil surface of theprojection objective, where the raster points are spaced apart from eachother in a two-dimensional array at predefined distances.

In a third step, ray trajectories of rays originating from therepresentative field points and passing through the raster points of thepupil raster are calculated for each of the representative field points.For that purpose, commercially available design programs such as CODE V,OSLO or ZEMAX may be used.

The pupil raster typically encompasses the entire utilized aperture ofthe projection objective, where the aperture determines the openingangle of ray bundles originating from each representative field point.In a pupil raster schematically illustrated in FIG. 4, the coordinatesof the raster points in the pupil surface are given in polar coordinatessuch that neighbouring raster points (i.e. raster points immediatelyadjacent to each other with a predefined distance there between) havethe same distance in an azimuthal direction. In such embodiments, anangular step width between neighbouring raster points in thecircumferential (azimuthal) direction is 10/3 degrees. The coordinatesof raster points in the radial direction (radial coordinate) correspondto aperture angles included between the respective rays and the opticalaxis. The angles are also denoted as “pupil angle” in this application.The absolute value of the sine of the pupil angle is stepwise increasedaccording to a square-root function between angle θ and a maximum angleand k_(max)=NA·β according to k_(i)=√{square root over (i/n)}k_(max),where i=0, 1, . . . , n, NA is the image-side numerical aperture of theprojection objective, and β is the magnification factor between objectfield and image field. By doing so, the pupil surface is subdivided intoraster fields (or raster cells) having substantially the same rasterfield area.

In a fourth step intersection points of the selected rays with theoptical surfaces within the projection objective are calculated for eachoptical surface (potentially excluding the last optical surfaceimmediately adjacent to the image surface in some cases).

In the subsequent steps pairs of directly neighbouring raster points inthe pupil surface are considered. Directly neighbouring raster pointsare characterized in that both raster points of the pair have either thesame azimuthal coordinate or the same pupil angle coordinate, while therespective other coordinates differ by one coordinate step in therespective coordinate direction according to a preselected step width.For each pair of neighbouring raster points, the difference quotient

$\begin{matrix}{g_{ij}^{f} = \frac{{x_{i}^{f} - x_{j}^{f}}}{{k_{i} - k_{j}}}} & (1)\end{matrix}$

is calculated, where f represents the number of the respective opticalsurface, and I, j represent the indices of neighbouring pupil rasterpoints. In equation (1), variables x and k are vectors. The componentsof vector x represent coordinates of a point an an optical surface inreal space. The components of vector k are direction sine valuesrepresenting the x, y and z coordinates of a unit vector pointing in thepropagation direction of a ray in the entrance pupil of the opticalsystem (i.e. in pupil space). Therefore, the difference quotient in (1)is a measure indicating the relation between a step with predefined stepwidth in pupil space (variable k) and a corresponding step width in realspace on the optical surface (variable x). With other words: thegradient parameter g^(f) _(ij) defined by the difference quotient ofequation (1) describes the degree of change of intersection points on arespective surface for a given difference in pupil coordinates. (Thedifference quotient of equation (1) is an approximation of adifferential quotient indicating that a finite step widths is typicallyused in the numerical calculation. The difference quotient of equation(1) becomes a differential quotient as the step width approaches zero).

Note that the numerical criterion given in equation (1) above is definedusing a linear gradient. More precisely, the ratio between areas of onegrid mesh element on the surface corresponding to a related one indirectional space could be controlled. Practically it shows, however,that controlling the linear gradient in nearly orthogonal directions issufficient in most cases to avoid local peaks of ray density.

In a further step, a gradient threshold value is defined, whichrepresents a minimum acceptable gradient between neighbouringintersection points on the optical surfaces. For example, a gradientthreshold value g^(f) _(ij) (min)=10 mm may be defined.

As a further illustration of the meaning of the gradient parameter FIG.5 shows the intersection points of rays of the pupil raster shown inFIG. 4 on a selected optical surface f for a selected representativefield point. It is evident, that the distribution of intersectionpoints, and as a consequence, the distribution of irradiance across theoptical surface, is not uniform since the local density of intersectionpoints is significantly larger in a high irradiance region HIRAD at thelower right part of the optical surface than in other parts of theoptical surface such as in the upper left part diametrically opposed tothe high irradiance region. However, no caustic conditions are given onthe optical surface since the sequence of intersection points inazimuthal and radial direction is the same as in the pupil surface(although the step width between neighbouring intersection pointsindicative of the amount of a irradiance in the respective area variessignificantly across the optical surface.) The situation shown in FIG. 5corresponds to value of 4.7 mm for the gradient parameter g^(f) _(ij).

In a next step, the minimum value of the gradient parameter iscalculated for each of the optical surfaces (optionally except for thelast optical surface). If the minimum value calculated for a specificoptical surface is found to be smaller than the gradient thresholdvalue, then the structural parameters of the projection objective areoptimized such that the minimum gradient on the respective opticalsurface increases until its value is equal to or larger than thegradient threshold value. For example, the optimization procedure mayinclude increasing the distance between the respective optical surfaceand a neighbouring field surface (such as the object surface, anintermediate image or the image surface). Alternatively, or in addition,the local inclination of the optical surface in the region of minimumgradient may be changed to increase the gradient calculated forneighbouring intersection points.

A corresponding radius R_(SUBEFF) of an “effective sub-aperture” can becalculated according to:

R _(SUBEFF)=Min(g ^(f) _(ij))*NA _(OBJ),  (2)

where Min(g^(f) _(ij)) is the minimum value of the difference quotientin equation (1) and NA_(OBJ) is the image side numerical aperture. Notethat a region with minimum value for the gradient parameter correspondsto a region with maximum geometrical ray density (and maximum effectiveirradiance).

As a final result of the optimization procedure, all optical surfaces ofthe projection objective are positioned and shaped such that thegradient parameter is equal to or larger than the gradient thresholdvalue. In terms of irradiance this desired property corresponds to thecondition that a normalized irradiance value IRRAD representing anirradiance normalized to an irradiance in an image surface of theprojection objective does not exceed a predefined irradiance thresholdvalue IRRTV on each optical surface of the projection objective (exceptfor the last optical surface directly adjacent to the image surface ofthe projection objective). Where the irradiance does not exceed anacceptable maximum value for the normalized irradiance, the opticalperformance of the projection objective becomes relatively insensitiveto potential imperfections on the optical surfaces, as described above.

For comparison, FIG. 6 shows the intersection points of rays of thepupil raster on a “virtual” system surface within a region where causticconditions occur for a field point. In the upper left part of thevirtual surface it is evident that the intersection points correspondingto pupil coordinates at the outer edge of the pupil lie closer to theintersection point of the axial ray than intersection pointscorresponding to raster points positioned somewhere between the opticalaxis and the outer edge of the pupil. With other words, rays originatingfrom a certain field point at different numerical aperture (representedby different radial coordinates in the pupil raster) intersect on therespective optical surface or in the vicinity thereof such that a raycorresponding to a larger aperture value in the pupil surface intersectswith the optical surface closer to the axial ray than rays correspondingto smaller aperture values. In this representation, the gradientparameter g^(f) _(ij) attains its minimum value 0 mm in the region ofthe optical surface which lies in a caustic region CAUSTIC (upper leftpart).

Employing this method systematically leads to optical designs where noneof the optical surfaces is positioned in a caustic region and/or in aregion with very small effective sub-apertures. Where such opticalsurfaces are avoided in an optical system, specifications with regard tosurface quality and/or contamination may be relaxed, therebyfacilitating manufacturing of an optical system.

FIG. 7 shows a catadioptric projection objective 100 observing thoseconditions. The projection objective is designed for a nominalUV-operating wavelength λ=193 nm. The specification is given in tables1, 1A. Projection objective 100 is designed to project an image of apattern on a reticle arranged in the planar object surface OS (objectplane) into the planar image surface IS (image plane) on a reducedscale, for example, 4:1, while creating exactly two real intermediateimages IMI1, IMI2. The rectangular effective object field OF and imagefield IF are off-axis, i.e. entirely outside the optical axis AX. Afirst refractive objective part OP1 is designed for imaging the patternin the object surface into the first intermediate image IMI1 at anenlarged scale. A second, catadioptric (refractive/reflective) objectivepart OP2 images the first intermediate image IMI1 into the secondintermediate image IMI2 at a magnification close to 1:(−1). A third,refractive objective part OP3 images the second intermediate image IMI2onto the image surface IS with a strong reduction ratio.

The path of the chief ray CR of an outer field point of the off-axisobject field OF is drawn bold in FIG. 7 in order to facilitate followingthe beam path of the projection beam. For the purpose of thisapplication, the term “chief ray” (also known as principal ray) denotesa ray running from an outermost field point (farthest away from theoptical axis) of the effectively used object field OF to the center ofthe entrance pupil. Due to the rotational symmetry of the system thechief ray may be chosen from an equivalent field point in the meridionalplane as shown in the figures for demonstration purposes. In projectionobjectives being essentially telecentric on the object side, the chiefray emanates from the object surface parallel or at a very small anglewith respect to the optical axis. The imaging process is furthercharacterized by the trajectory of marginal rays. A “marginal ray” asused herein is a ray running from an axial object field point (fieldpoint on the optical axis) to the edge of an aperture stop. Thatmarginal ray may not contribute to image formation due to vignettingwhen an off-axis effective object field is used. The chief ray andmarginal ray are chosen to characterize optical properties of theprojection objectives. The angles included between such selected raysand the optical axis at a given axial position are denoted as “chief rayangle” (CRA) and “marginal ray angle” (MRA), respectively. The radialdistance between such selected rays and the optical axis at a givenaxial position are denoted as “chief ray height” (CRH) and “marginal rayheight” (MRH), respectively.

Three mutually conjugated pupil surfaces P1, P2 and P3 are formed atpositions where the chief ray CR intersects the optical axis. A firstpupil surface P1 is formed in the first objective part between objectsurface and first intermediate image, a second pupil surface P2 isformed in the second objective part between first and secondintermediate image, and a third pupil surface P3 is formed in the thirdobjective part between second intermediate image and the image surfaceIS.

The second objective part OP2 includes a single concave mirror CM. Afirst planar folding mirror FM1 is arranged optically close to the firstintermediate image IMI1 at an angle of 45° to the optical axis AX suchthat it reflects the radiation coming from the object surface in thedirection of the concave mirror CM. A second folding mirror FM2, havinga planar mirror surface aligned at right angles to the planar mirrorsurface of the first folding mirror, reflects the radiation coming fromthe concave mirror CM in the direction of the image surface, which isparallel to the object surface.

The folding mirrors FM1, FM2 are each located in the optical vicinity ofan intermediate image, so that the etendue (geometrical flux) is keptsmall. The intermediate images are optionally not located on the planarmirror surfaces, which results in a finite minimum distance between theintermediate image and the optically closest mirror surface. This is toensure that any faults in the mirror surface, such as scratches orimpurities, are not imaged sharply onto the image surface.

The first objective part OP1 includes two lens groups LG1, LG2 each withpositive refractive power on either side of the first pupil surface P1.First lens group LG1 is designed to image the telecentric entrance pupilof the projection objective into the first pupil surface P1, therebyacting in the manner of a Fourier lens group performing a single Fouriertransformation. This Fourier transformation leads to a relatively smallmaximum chief ray angle CRA_(P1) in the order of 17° at the first pupilsurface. As a consequence, according to the Lagrange invariant, theoptically free diameter of the first pupil is relatively large,indicated by a diameter D₁=145 mm of the radiation beam in the firstpupil surface.

The relatively small chief ray angle along with the large pupil diametercorresponds to a relatively large axial extension of a pupil space PS.For the purpose of this application the pupil space is defined as aregion where the marginal ray height MRH is substantially greater thanthe chief ray height CRH such that the condition RHR<|B|<<1 is fulfilledfor the ray height ratio RHR=CRH/MRH. The upper limit B of the rayheight ratio may be smaller than 0.4 or smaller than 0.3 or smaller than0.2, for example. If this condition is fulfilled, a correction appliedin the pupil space will have an essentially field-constant effect. Theaxial extension of the pupil space is increased as the chief ray angleat the respective pupil is reduced. In such embodiments, the conditionRHR<0.3 is fulfilled.

In such embodiments, the pupil space PS includes the first lens L1-6 ofLG2 immediately downstream of the pupil surface (biconvex lens) andextends up to the subsequent lens L1-7 on the image-side of the pupilsurface and up to the biconvex positive lens L1-5 immediately upstreamof the first pupil surface. Free spaces FS1 (=41 mm) and FS2 (=62 mm),each having an axial extension of at least 40 mm allowing to place oneor more thin correcting elements into the free space are formed oneither side of the pupil surface P1 within the pupil space PS, i.e.optically close to the pupil surface. Therefore, such embodiments allowintroducing one or more correcting elements optically close to the firstpupil surface P1 in order to obtain correcting effects which areessentially the same for all field points of the field (field-constantcorrection).

A parallel plate PP is positioned in the pupil space PS at the firstpupil surface P1 where the condition RHR≈0 is fulfilled. The parallelplate is part of the original design of the projection objective and mayserve as a placeholder for a correcting element which may also be formedessentially as a plane parallel plate having the same thickness andmaterial, where at least one surface has an aspheric shape.

As mentioned above, in high-aperture projection objectives typicallyused in microlithography relatively small real and/or effectivesub-apertures of ray bundles may occur at certain optical surfaces, forexample optical surfaces close to a field surface. For example, in FIG.7, the first folding mirror FM1 and the second folding mirror FM2 areboth positioned optically close to a neighboring intermediate image IMI1and IMI2, respectively such that relatively small real sub-aperturesoccur on those folding mirrors. In general, the irradiance on an opticalsurface increases as the size of sub-apertures decreases. Whererelatively large values of irradiance occur on an optical surface, thoseoptical surfaces may be relatively important with respect to surfaceimperfections, such as scratches and contamination. Also, causticconditions may occur on certain optical surfaces within the opticalsystem, particularly close to field surfaces. Where an optical surfacelies in a caustic region, the irradiance may become divergent.

Due to these effects the specifications with regard to surface qualityand contamination is desirablyy kept particularly severe in opticalsystems having optical surfaces in regions of small sub-apertures and/orin regions where caustic conditions occur. On the other hand, if suchsurfaces are avoided in an optical system, specifications with regard tosurface quality and/or contamination may be relaxed, therebyfacilitating manufacturing of an optical system.

In the manufacture of the embodiment of FIG. 7, particular emphasis wasplaced on the correaction of the ray paths such that the first andsecond folding mirrors FM1, FM2 are placed well apart from theintermediate images to obtain relatively large sub-apertures and alsowith respect of control of caustic conditions such that no causticcondition occurs on either of the folding mirrors FM1 and FM2. This isdemonstrated qualitatively in FIG. 8, which shows footprints of 18selected field points around the edge of a rectangular field on thefirst folding mirror FM1 and the second folding mirror FM2. In eachfootprint, ray bundles are shown in 10 equidistant aperture steps. It isevident that the substantially elliptic lines corresponding to differentaperture ray bundles originating from the same field point do notintersect, but are interleaved without intersection of the foldingmirrors. This indicates that both the first and the second foldingmirror are in regions without caustic conditions, i.e. in “caustic-free”regions. Also, the biconvex positive lens arranged in a double-pathregion geometrically between the folding mirrors FM1, FM2 and theconcave mirror and optically relatively close to the first and secondintermediate images is in a caustic-free region. As a consequence, theembodiment of FIG. 7 is relatively tolerant with regard to surfaceimperfections and/or contamination on the optical surfaces close to theintermediate images IMI1, IMI2.

The above description of certain embodiments has been given by way ofexample. The individual features may be implemented either alone or incombination as embodiments of the disclosure, or may be implemented inother fields of application. Further, they may represent advantageousembodiments that are protectable in their own right, for whichprotection is claimed in the application as filed or for whichprotection will be claimed during pendency of the application. From thedisclosure given, those skilled in the art will not only understand thepresent disclosure and its attendant advantages, but will also findapparent various changes and modifications to the structures and methodsdisclosed. The applicant seeks, therefore, to cover all such changes andmodifications as fall within the spirit and scope of the disclosure, asdefined by the appended claims, and equivalents thereof.

The contents of all the claims is made part of this description byreference.

TABLE 1 NA = 1.36; λ = 193 nm; image field height y′ = 15.3 mm THICK-MATE- SEMI- SURF RADIUS NESS RIAL INDEX DIAM. 0 0.000000 35.011188 61.01 382.185274 19.022510 SILUV 1.560491 76.3 2 3355.596936 28.816162 78.23 477.218860 22.631304 SILUV 1.560491 88.5 4 −922.196146 38.663099 89.65 −1118.102672 50.000005 SILUV 1.560491 94.1 6 −166.502079 68.19352296.5 7 −129.699842 10.289512 SILUV 1.560491 77.3 8 −568.486045 1.40450881.1 9 360.871595 36.412765 SILUV 1.560491 83.7 10 −221.344939 41.31618684.0 11 0.000000 10.000000 SILUV 1.560491 72.7 12 0.000000 4.999963 74.513 558.300871 22.827559 SILUV 1.560491 78.0 14 −433.013634 62.17356479.5 15 441.363089 31.099429 SILUV 1.560491 89.5 16 −341.473462 1.17028189.5 17 1222.280168 28.519264 SILUV 1.560491 87.1 18 −247.0395068.650761 85.9 19 −185.932756 10.099134 SILUV 1.560491 84.0 20 182.28621855.304594 81.0 21 −119.613521 10.814913 SILUV 1.560491 81.8 224853.350949 2.068171 100.7 23 496.760047 50.132029 SILUV 1.560491 111.124 −245.672304 88.351331 114.4 25 444.836588 64.064520 SILUV 1.560491145.2 26 −295.503989 76.306203 145.2 27 0.000000 −314.280203 REFL 132.828 −247.137511 −77.271453 SILUV 1.560491 160.0 29 810.350730 −208.473351158.0 30 294.775031 −12.500000 SILUV 1.560491 88.1 31 −286.941223−102.432781 84.7 32 180.414660 −12.500000 SILUV 1.560491 87.2 3310035.763024 −26.892770 94.5 34 194.482349 26.892770 REFL 96.0 3510035.763024 12.500000 SILUV 1.560491 94.5 36 180.414660 102.432781 87.237 −286.941223 12.500000 SILUV 1.560491 84.7 38 294.775031 208.47335188.1 39 810.350730 77.271453 SILUV 1.560491 158.0 40 −247.137511314.280047 160.0 41 0.000000 −78.000672 REFL 133.2 42 −339.895111−48.598415 SILUV 1.560491 146.4 43 1373.456750 −0.999276 145.7 44−504.213001 −36.238195 SILUV 1.560491 142.0 45 5356.202306 −0.999341139.7 46 −140.155457 −64.600486 SILUV 1.560491 114.8 47 −334.057880−20.315663 100.9 48 6443.720921 −9.999173 SILUV 1.560491 96.9 49−92.325569 −33.314435 73.6 50 −368.587379 −9.998836 SILUV 1.560491 73.051 −149.804097 −48.568442 69.7 52 102.768893 −9.999251 SILUV 1.56049169.8 53 −268.102298 −17.334160 86.4 54 −894.642180 −45.003218 SILUV1.560491 91.1 55 341.239295 −3.399868 107.0 56 555.007734 −98.611795SILUV 1.560491 110.1 57 161.562349 −0.999537 135.5 58 1424.973978−41.439862 SILUV 1.560491 148.9 59 240.839763 −0.999688 152.2 60−861.203459 −50.810112 SILUV 1.560491 158.2 61 771.781511 −33.917678160.0 62 −546.418872 −47.441425 SILUV 1.560491 160.0 63 1544.67166518.607639 159.4 64 0.000000 −19.607228 160.7 65 −280.711459 −56.538959SILUV 1.560491 148.9 66 −2833.791703 −0.998656 145.5 67 −151.439302−51.205187 SILUV 1.560491 109.5 68 −2634.011137 −1.000000 101.4 69−60.618256 −52.074212 SILUV 1.560491 54.6 70 0.000000 −3.000000 H2OV1931.436823 23.6 71 0.000000 0.000000 15.3

TABLE 1A ASPHERIC CONSTANTS SRF 2 8 23 26 29 K 0 0 0 0 0 C1 1.347468E−086.034781E−08 −1.016331E−08 1.654839E−08 −1.366914E−08 C2 2.207912E−121.030721E−12 9.975098E−13 1.248738E−13 1.191027E−13 C3 9.965819E−174.363266E−16 −6.682385E−17 −2.875416E−19 −1.501877E−18 C4 3.306256E−20−1.299667E−19 2.186818E−21 −l.140427E−23 3.212707E−23 C5 −6.720858E−241.514107E−23 −2.369614E−26 −9.698834E−29 −7.689882E−28 C6 5.149578E−28−7.081928E−28 −3.385824E−31 7.791686E−33 9.534303E−33 SRF 33 35 39 43 44K 0 0 0 0 0 C1 3.177312E−08 3.177312E−08 −1.366914E−08 −2.403138E−09−4.337241E−09 C2 −1.077719E−12 −1.077719E−12 1.191027E−13 −9.726816E−14−1.770230E−13 C3 5.276309E−17 5.276309E−17 −1.501877E−18 −3.883610E−18−6.575209E−18 C4 −2.529805E−21 −2.529805E−21 3.212707E−23 1.766230E−227.684192E−23 C5 1.307247E−25 1.307247E−25 −7.689882E−28 −3.969746E−27−8.791789E−28 C6 −7.341513E−30 −7.341513E−30 9.534303E−33 6.671253E−326.418850E−32 SRF 54 56 58 60 63 K 0 0 0 0 0 C1 3.845765E−08 5.917411E−083.947900E−08 −1.247993E−10 1.795292E−08 C2 3.149540E−12 −1.711396E−12−1.804073E−13 −1.251219E−13 −1.289060E−12 C3 −1.246913E−16 −3.877057E−17−3.154989E−17 2.971143E−17 5.175936E−17 C4 6.730427E−21 −6.143291E−222.071680E−21 −8.028081E−22 −1.070499E−21 C5 −1.009901E−24 5.363788E−25−8.838538E−26 2.050122E−26 1.388166E−26 C6 −3.983455E−29 −2.016608E−291.721616E−30 −3.004535E−31 −1.098382E−31 SRF 66 68 K 0 0 C1 2.376124E−08−2.061112E−08 C2 −4.937362E−13 −2.944479E−12 C3 2.984424E−172.901078E−16 C4 −1.965712E−21 −2.145724E−20 C5 5.583512E−26 1.078059E−24C6 −5.689719E−31 −3.014279E−29

1. A method, comprising: computing a numerical value for each of aplurality of merit function components based on a corresponding featureof a preliminary design of a projection objective, wherein: each of theplurality of merit function components corresponds to a respectivequality parameter; and one of the plurality of merit function componentsrequires that, for each optical surface of the projection objectiveexcept a last optical surface of the projection objective, an effectiveirradiance of the optical surface normalized to an effective irradiancein an image surface of the projection objective does not exceed athreshold value; computing from the merit function components an overallmerit function expressible in numerical terms that reflect correspondingquality parameters; successively varying at least one structuralparameter of the projection objective and recomputing a resultingoverall merit function value with each successive variation until theresulting overall merit function reaches a predetermined acceptablevalue; and obtaining the structural parameters of the optimizedprojection objective having the predetermined acceptable value for theresulting overall merit function.
 2. The method of claim 1, furthercomprising using the structural parameters to make the projectionobjective.
 3. A method according to claim 1, further comprising:calculating a position and an extent of potential caustic regions withinthe projection objective; and optimizing the structural parameters ofthe projection objective such that no optical surface is positionedinside a caustic region.
 4. A method according to claim 1, furthercomprising: defining a number of representative field points; defining apupil raster representing an array of mutually spaced apart rasterpoints in a pupil surface of the projection objective; calculating, foreach of the representative field points, ray trajectories of raysoriginating from the representative field points and passing through theraster points of the pupil raster; calculating, for each opticalsurface, intersection points of the rays with the optical surface;calculating for each optical surface a plurality of gradient parametersrepresenting respective gradients between intersection pointscorresponding to neighbouring raster points arranged directly adjacentto each other; defining a gradient threshold value representing aminimum acceptable gradient between neighbouring intersection points;and optimizing structural parameters of the projection objective suchthat the gradient parameter does not fall below the gradient thresholdvalue for each optical surface of the projection objective except forthe last optical surface.
 5. A method according to claim 4, wherein thepupil raster is defined such that the pupil surface is subdivided intoraster fields having substantially the same raster field area.
 6. Amethod according to claim 4, wherein the pupil raster is defined inpolar coordinates such that neighbouring raster points have the samedistance in an azimuthal direction and a pupil angle k varies in stepsbetween 0 and k_(max)=NA·β according to k_(i)=√{square root over(i/n)}k_(max), where i=0, 1, . . . , n, NA is the image-side numericalaperture of the projection objective, and β is the magnification factorbetween object field and image field.
 7. The method according to claim1, further comprising: defining a number of representative field points;calculating ray bundles originating from the field points andintersection zones of the ray bundles with optical surfaces, where anintersection zone of a ray bundle with an optical surface defines a realsub-aperture having a real sub-aperture size defined by the area of theintersection zone; defining a sub-aperture size threshold value; andoptimizing the structure parameters of the projection objective suchthat the real sub-aperture size for selected field points does not fallbelow the sub-aperture size threshold value for all optical surfaces ofthe projection objective except for a last optical surface directlyadjacent to an image surface of the projection objective.
 8. The methodaccording to claim 1, wherein the projection objective is a catadioptricprojection objective designed to image an off-axis object field arrangedin the object surface into an off-axis image field arranged in the imagesurface, the projection objective comprising: at least one concavemirror; at least one intermediate image; and at least one folding mirrorarranged to deflect radiation coming from an object surface towards theconcave mirror or arranged to deflect radiation coming from the concavemirror towards the image surface.
 9. The method according to claim 8,wherein the optical elements form: a first refractive objective partgenerating a first intermediate image from radiation coming from theobject surface and including a first pupil surface; a second objectivepart including the at least one concave mirror imaging the firstintermediate image into a second intermediate image and including asecond pupil surface optically conjugated to the first pupil surface;and a third refractive objective part imaging the second intermediateimage onto the image surface and including a third pupil surfaceoptically conjugated to the first and second pupil surfaces.
 10. Themethod according to claim 9, wherein the projection objective hasexactly two intermediate images and/or wherein the second objective parthas exactly one concave mirror and the projection objective has a firstfolding mirror to deflect radiation coming from the object surface inthe direction of the concave mirror, and a second folding mirror todeflect radiation coming from the concave mirror in the direction of theimage surface and/or wherein the projection objective is designed forimmersion lithography at NA>1.
 11. The method of claim 1, wherein theprojection objective is designed to be used in microlithography.
 12. Anoptical system, comprising: a catadioptric projection objective havingan object surface and an image surface, the catadioptric projectionobjective comprising: at least one concave mirror; at least oneintermediate image; and at least one folding mirror arranged to deflectradiation coming from an object surface of the optical system towardsthe concave mirror or arranged to deflect radiation coming from theconcave mirror towards an image surface of the optical system, wherein:optical elements with optical surfaces of the catadioptric projectionobjective are configured to image an off-axis object field arranged inthe object surface into an off-axis image field arranged in the imagesurface; and structural parameters of the catadioptric projectionobjective are adjusted such that no optical surface of the catadioptricprojection objective is positioned inside a caustic region.
 13. Theoptical system according to claim 12, wherein the optical elements form:a first refractive objective part generating a first intermediate imagefrom radiation coming from the object surface and including a firstpupil surface; a second objective part including the at least oneconcave mirror imaging the first intermediate image into a secondintermediate image and including a second pupil surface opticallyconjugated to the first pupil surface; and a third refractive objectivepart imaging the second intermediate image onto the image surface andincluding a third pupil surface optically conjugated to the first andsecond pupil surfaces.
 14. The optical system according to claim 13,wherein the projection objective has exactly two intermediate imagesand/or wherein the second objective part has exactly one concave mirrorand the projection objective has a first folding mirror to deflectradiation coming from the object surface in the direction of the concavemirror, and a second folding mirror configured to deflect radiationcoming from the concave mirror in the direction of the image surfaceand/or wherein the projection objective is designed for immersionlithography at NA>1.
 15. The optical system according to claim 12,wherein the at least one folding mirror is a planar mirror.
 16. Asystem, comprising: the optical system of claim 12, wherein the systemis a projection exposure machine.
 17. A method, comprising: designing aprojection objective using a merit function component that, for eachoptical surface of the projection objective, compares an effectiveirradiance of the optical surface normalized to an effective irradiancein an image surface of the projection objective.
 18. The method of claim17, wherein the merit function component requires that, for each opticalsurface of the projection objective except a last optical surface of theprojection objective, the effective irradiance of the optical surfacenormalized to an effective irradiance in an image surface of theprojection objective does not exceed a threshold value.
 19. The methodof claim 17, further comprising, after designing the projectionobjective, making the projection objective.
 20. The method of claim 17,wherein the projection objective is designed to be used inmicrolithography.